Compositions for use in the wet-end of papermaking

ABSTRACT

A composition for use as a strengthening agent in the wet-end of papermaking which comprises a surface modified non-wood plant fiber and a starch component.

TECHNICAL FIELD

The present invention relates to novel compositions for use in the wet-end of papermaking. In particular, it relates to compositions comprising both starch and a non-wood plant fiber, and to their use for increasing the dry strength of paper products.

BACKGROUND OF THE INVENTION

During the production of paper, cellulose is mixed with water and pulped to form a slurry (or “cellulosic pulp”). The slurry is then applied to a screen to orient the cellulose fibers and form a paper web. Excess liquid is allowed to drain off. This process is referred to as “the wet-end” of papermaking. The web is then passed through a press section, to further reduce water content, followed by a drying step, typically using steam- or oil-heated drying cylinders. The dried paper web may then be impregnated with a sizing composition or coated before being cut to size.

The strength of the finished paper product will depend on a number of factors including, for example, the use and nature of any sizing or coating compositions, the use and nature of any wet-end additives and, of course, the nature of the pulp itself. Virgin wood pulp typically gives a stronger paper than pulp obtained from recycled paper. In fact, the strength of a paper product will diminish with every re-pulping cycle. At the same time, there is pressure in the industry to move towards greater use of recycled materials, both for environmental reasons and for cost reasons. There is therefore a clear need to identify ways of increasing—or at least maintaining—dry strength in paper products containing recycled materials.

One possible solution would be the use of dry strength additives in the wet-end of the paper production process. Dry strength additives for use in the wet-end have typically consisted of cationic starches. Cationic starch normally interacts with anionic cellulose fibers in the pulp to form complex matrices, thereby strengthening the paper web. Unfortunately, such cationic starches have been found to be rather ineffective when used in combination with recycled materials. It is indeed believed that recycled materials contain high levels of anionic trash which effectively “saturate” the cationic starch and therefore limit its interaction with the anionic cellulose fibers. Papermaking in closed circuits will lead to an accumulation of these trash products together with salts which impair the electrostatic interactions between anionic fibers and the cationic starch. As a result, the use of cationic starches in combination with recycled materials has a lesser ability to effectively increase dry strength.

A number of synthetic alternatives to starch-based strength additives have been proposed. These have included, for instance, polyacrylamide and polyvinylamine and are typically more efficient, even when used in combination with recycled materials. Unfortunately, they are also prohibitively expensive and may be considered undesirable from an environmental perspective.

There is therefore a clear need in the art for the development of a natural additive which can be used in the wet-end of papermaking to maintain or even increase dry strength levels in paper products even if they include recycled pulp materials and even when produced in closed circuits. The present invention provides such an additive.

STATEMENTS OF THE INVENTION

According to a first aspect of the present invention, there is provided a composition for use as a strengthening agent in the wet-end of papermaking which comprises a surface modified non-wood plant fiber and a starch component, wherein the plant fiber and starch component are preferably linked.

According to a further aspect of the present invention, there is provided a method of producing the above composition and a method of producing paper products with such a composition. Paper products obtainable by such a process are provided in another aspect of the invention.

According to an additional aspect of the present invention, there is provided the use of a surface modified non-wood plant fiber to increase the dry strength of paper, preferably paper comprising recycled materials and/or paper produced at conductivity levels above 4 mS/cm.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a composition for use as a strengthening agent in the wet-end of papermaking which comprises a surface modified non-wood plant fiber and a starch component.

Plant Fiber

The term “non-wood plant fiber” as used herein refers to any fibrous material which is not derived from wood, wherein “wood” will be understood to have its common dictionary definition (i.e. the hard, fibrous substance which makes up most of the trunk and branches of a tree). Examples of suitable non-wood plant fibers (simply “plant fibers” herein) will be apparent to a person skilled in the art. They include, but are not limited to, cereal fibers, seed fibers, legume fibers, fruit fibers, algae fibers and mixtures of two or more thereof.

The plant fibers may be provided as such or in the form of a fibrous material. Examples of suitable fibrous materials may include, for instance, cereal bran (e.g. wheat bran), vegetable hulls (such as soy hulls and/or pea hulls), bagasse, corn stover, straw, switch grass, citrus fibers (such as citrus pulp fiber), seaweed residue and mixtures of two or more thereof. The fibrous material will preferably comprise fibers in an amount of at least 50% by weight, on a dry weight basis, more preferably in an amount of at least 75% by weight. Certain fibrous materials may also comprise non-fibrous elements such as proteins or starch. By way of example, soy hulls typically comprise about 10% by weight protein and about 20% by weight starch, in addition to their fibrous content. Thus, according to at least one embodiment, the fibrous material will comprise approximately 50-85% fibers by weight, on a dry weight basis.

The plant fibers will preferably be anionic (meaning that it will preferably have a net negative charge). According to certain embodiments, they will have a streaming zeta potential (SZP) of between 0 and −30 mV, preferably of between −1 and −20 mV, more preferably of between −1 and −15 mV. According to another embodiment, they will preferably have an anionicity of 10 to 2000 μg/g, more preferably of 100 to 1000 μeq/g (expressed per gram of dry substance, at pH 7).

The plant fibers will advantageously be surface modified such that their ability to interact (mechanically or electrostatically) with cellulose fibers in a cellulosic pulp is increased. By way of example only, the fibers may be modified to have a greater overall or specific surface area.

As will be understood by a person skilled in the art, the term “surface modification” or “surface modified” will not be limited to modifications having an effect only on the surface of the fibers. Indeed, the modification(s) may also result in changes to other properties (e.g. the internal structure) of the fibers. Surface modification may be achieved, for example, by milling, heat treatment (e.g. cooking and/or extrusion), chemical modification or any combination thereof. Thus, according to certain embodiments, the plant fibers may be provided in the form of a powder or flakes.

The composition of the present invention also comprises a starch component.

Starch Component

Although not wishing to be bound by theory, it is believed that the starch component will increase the plant fiber's ability to interact with the cellulosic pulp, acting almost as an adhesive between the plant fibers and the cellulose fibers.

The term “starch component” as used herein may refer to one or more native starches, one or more modified starches, one or more starch derivatives (such as a dextrin) or mixtures thereof. The starch component may be derived from starches of any type and any origin. For example it may comprise waxy and/or non-waxy starches, and it may be derived from wheat, corn, potato, tapioca, pea or any other available starch source, and mixtures thereof.

The starch component may have been subjected to one or more modifications, including chemical, enzymatic and/or heat-based modifications. Thus, the starch component may comprise, for instance, one or more cross-linked, etherified, esterified, hydroxypropylated, and/or thinned starches. Preferably, the starch component will be anionic or cationic, i.e. having a net negative or a net positive charge. More preferably, the starch component will be cationic. According to one particular embodiment, the starch component will consist of one or more cationic starches.

According to one possible embodiment, some or all of the starch component may be contributed as part of a fibrous material. As described above, the plant fibers may be provided in the form of a fibrous material which may contain certain non-fibrous elements such as starch. The fibrous side-stream recovered from wheat processing, for example, may contain up to 50% starch by weight. If using this fibrous side-stream as the source of plant fibers, it may not be necessary to add any starch to the composition (or to only add a small amount of starch).

The starch component will preferably be “activated”, meaning that it will be rendered capable of forming a dispersion in cold water (i.e. at about 20-25° C.). Preferably, the starch component will be solubilised, meaning that it will be formed into (or capable of forming) a stable colloidal dispersion. This can be achieved, for example, by cooking (heat treating, steam treating, etc.), by thinning (e.g. with acid), by extrusion, by pregelatinization, by roll-drying or by any combination thereof. According to one possible embodiment, the one or more starch components will comprise one or more cold water soluble cationic starches.

When used, the composition of the present invention will preferably comprise the starch component in an amount of 1-90% by weight, preferably of 1-50% by weight, more preferably of 5-30% by weight, for example 20-25% by weight, based on the total dry weight of the composition. Ideally, the composition will comprise the plant fibers and the starch component in a weight ratio, based on dry weight, of from 1:20 to 20:1, preferably of from 1:1 to 10:1.

Preferably, the starch component and non-wood plant fibers will be linked. The term “linked” as used herein may refer to a direct or indirect link, whether through absorption, electrostatic interaction, chemical bond or any other means, which allows at least part of the starch component to be retained on the surface of some or all of the plant fibers. In a preferred embodiment, at least part of the cationic starch component will be linked to the anionic plant fibers through a direct electrostatic link. Alternatively, an anionic starch may be linked to the anionic plant fibers through a cationic bridge (e.g. a cationic trivalent or divalent metal ion such as calcium or aluminium, or cationic starch).

Advantageously, the composition of the present invention will comprise up to 100% by weight, based on total dry weight, of the plant fibers and starch component. Preferably, it will comprise 80-100% by weight of the plant fibers and starch component, more preferably 90-100% by weight, based on total dry weight.

Other Optional Ingredients

The composition of the present invention may comprise one or more further optional ingredients. These will preferably be selected from wet-end additives well known to a person skilled in the art. They may include, for instance, hardeners, flowability improvers, lubricants, antifoamers, releasing agents, optical brighteners, preservatives, yellowing inhibitors, ultraviolet absorbers, antioxidants, insolubilisers, antistatic agents, pH regulators, water-resisting agents, wet strength agents, sizing agents, dewatering aids, grease and oil resistance additives and combinations of two or more thereof.

The amount of each of these additives to be included, if at all, will be determined in accordance with standard practice and with the desired properties of the final paper product in mind.

Advantageously, the composition of the present invention will not need to include—and will therefore preferably not include—any synthetic dry strength additives. In fact, the composition of the present invention will preferably include less than 5%, preferably less than 2%, more preferably less than 1% by weight, based on total dry weight, of any synthetic additives, wherein “synthetic additives” will be understood as referring to non-naturally occurring chemical additives such as polyacrylamide, polyvinylamine, melamine resins, urea formaldehyde resins and so on.

The composition of the present invention will preferably be provided in the form of a powder. Alternatively, it may also be provided in the form of a slurry or of an aqueous composition. As such, it may have a total dry substance of between 10% and 98% by weight, preferably of between 50% and 98% by weight, more preferably of between 70% and 95% by weight, based on the total weight of the composition.

Strengthening Agent

The composition of the present invention is intended for use as a strengthening agent. In particular, when used in the manufacture of a paper product, it can be used to maintain or increase the product's dry strength (as measured for example by standard CMT, SCT and/or Burst tests). It is also believed that compositions of the present invention will contribute to a good or improved wet strength, that is to a good or improved strength for the wet paper web during the papermaking process, as described in more detail below.

Advantageously, the composition of the present invention may be used to increase the wet and/or dry strength of paper products containing recycled materials, especially when compared to cationic starch alone. The compositions of the present invention are believed to perform at least as well as synthetic agents such as polyacrylamide and/or polyvinylamine. The composition of the present invention may also be used to increase the wet and/or dry strength of paper products produced at conductivity levels above 4 mS/cm.

A further advantage of the composition of the present invention is that it is not detrimental to other important properties of the papermaking process such as retention (the retention of cellulose fibers on the web) or dewatering (the ability of water in the paper pulp to be easily removed). In some instances, it is even believed that the composition of the invention may contribute to an improvement in these properties (i.e. increased retention and increased dewatering).

Process of Producing the Composition

The present invention further provides a process for the production of the above composition. In particular, the present invention provides a process for the production of a composition for use as a strengthening agent in the wet end of paper making which comprises:

(a) providing a composition comprising a plant fiber and a starch component; and

(b) surface modifying at least the plant fiber; or:

(a) providing a composition comprising a plant fiber;

(b) surface modifying the plant fiber; and

(c) mixing the modified plant fiber composition with a starch component.

Where it is desired or intended that the starch component be cationic or anionic, cationisation or anionisation of the starch component may be performed prior to or during contact with the plant fiber. Thus, for example, where it is desired that the starch component be cationic, the process of the present invention may comprise the following steps:

(a) providing a composition comprising a plant fiber and a starch component;

(b) surface modifying at least the plant fiber; and

(c) cationizing the starch component (wherein b and c may be performed in any order); or:

(a) providing a composition comprising a plant fiber and a cationic starch component; and

(b) surface modifying at least the plant fiber; or:

(a) providing a composition comprising a plant fiber;

(b) surface modifying the plant fiber; and

(c) mixing the modified plant fiber composition with a cationic starch component; or:

(a) providing a composition comprising a surface modified plant fiber and a starch component; and

(b) cationizing the starch component.

Whether performed together or separately, activation of the plant fiber (through surface modification) and/or starch component may comprise, by way of example only: heat treatment (e.g. cooking, jet cooking, dry or semi-dry heat treatment, extrusion, roll-drying . . . ), mechanical treatment (e.g. dry milling or wet milling), chemical treatment (e.g. oxidation) and/or, for the starch component at least, pregelatinization. Preferably, the plant fiber and/or starch component will both be cooked. The plant fiber will preferably also be milled. If milled, the plant fiber will preferably be milled to an average particle size (D₅₀) in the range of 30-500 μm, preferably in the range of 30-200 μm, more preferably in the range of 30-100 μm. According to one particular embodiment, cooking may consist of jet cooking, e.g. at a temperature in the range of 80-180° C. or of 100-140° C.

The process of the present invention will preferably further comprise the step of forming a link (or allowing a link to form) between the plant fiber and starch component. As described above, this may be a direct or indirect link. Preferably, the plant fiber and starch component with be linked by electrostatic interaction. Thus, according to one possible embodiment, the process of the present invention will include the step of contacting the plant fiber and starch component in water. This will advantageously be done prior to use of the composition in the wet-end of papermaking, i.e. in clean water, to prevent the starch component from interacting with non-fibrous materials. Thus, “clean water” will be understood as having its ordinary meaning, that is: it does not refer to waste water or water with high levels of trash (i.e. white water). It need not refer to sterile or de-ionized water either (although of course such “hyper-clean” waters could be used). Instead it will typically refer to fresh water or simple “tap water”.

Thus, according to one particular embodiment of the present invention, there is provided a process for the production of a composition for use as a strengthening agent in the wet end of paper making which comprises:

(a) providing a surface modified plant fiber and a cationic, preferably cold water soluble, starch component; and

(b) mixing the plant fiber and starch component in clean water such that the plant fiber and starch component become linked, preferably by electrostatic interaction.

According to a preferred embodiment, the starch component and plant fiber will be brought into contact in clean water and then cooked to encourage the formation of links between these two components (i.e. to encourage them to interact). Thus, for example, according to a preferred embodiment, the present invention provides a process for the production of a composition for use as a strengthening agent in the wet end of paper making which comprises:

(a) providing a plant fiber and a cationic starch component;

(b) mixing the plant fiber and starch component in clean water; and

(c) heating the aqueous mixture of step (b), preferably at 80-180° C.

Preferably, the plant fiber and starch component will be brought into contact at high concentrations. By “high concentrations” it is meant that the plant fiber and starch component will preferably be mixed at 1-30% dry solids, more preferably at 5-20% dry solids, although, of course, in certain embodiments, the dry solids could be much higher. Without wishing to be bound by theory, it is believed that such concentrations will encourage interactions between the plant fibers and starch component. Of course, if necessary before use, the composition may be diluted and/or further optional ingredients may be added to it.

Products and Methods of Production

Compositions obtainable by this process, together with their use as strengthening agents and paper products made with them, are all part of the present invention. In particular, the present invention provides a method of producing a paper product comprising the steps of:

(a) bringing a composition as described above (or produced according to one of the above processes) into contact with a cellulosic pulp; and

(c) producing a paper product from the pulp obtained in step (a).

The term “cellulosic pulp” refers to an aqueous suspension of cellulosic fibers as typically used in the paper industry for the production of paper products. It will be understood as including any type of pulp suitable for use in the manufacture of paper products including, for instance, paper sheets, board (cardboard or corrugated board), packaging or case materials and so on. It may include virgin wood pulp, pulp from recycled materials, mechanical pulp, etc. The pulp may be bleached or unbleached and it may or may not include recycled materials.

Preferably, the pulp will comprise recycled materials (i.e. recycled cellulose fibers). Recycled materials may include any kind of recovered, waste or scrap paper products (“waste products”) which are re-pulped for further use. Examples of suitable waste products may include, for example, mill broke, pre-consumer waste and/or post consumer waste. The recycled materials may or may not be de-inked, bleached or treated in any other way before use and they may include materials that have already been recycled one or more times. According to one possible embodiment, the pulp may consist entirely of recycled materials. Preferably, it will comprise at least 50% recycled materials by weight, more preferably at lest 75% by weight.

Without wishing to be bound by theory, it is believed that the composition of the present invention is particularly useful in the production of paper products comprising recycled materials. In particular, it is believed that the composition of the present invention will provide better wet and dry strength properties in the production of paper products comprising recycled materials than cationic starches alone (and at least equivalent properties to their synthetic alternatives), without negatively affecting water drainage, retention or other important parameters.

According to one particular embodiment, the composition of the present invention will be used such that the composition of the present invention is brought into contact with the cellulosic pulp in an amount of 0.2 to 20%, preferably 0.5 to 10%, more preferably 2-6% by weight, based on the dry weight of the pulp. Expressed in a different way, paper products of the present invention will preferably comprise surface modified plant fibers and cellulose fibers in a weight ratio of 1:500 to 1:4, more preferably of 1:100 to 1:20, more preferably of 1:60 to 1:30.

The present invention will now be described in more detail by way of the following, non-limiting examples.

EXAMPLES

A number of tests were carried out using 100% recycled OCC paper with a conductivity value of 4.7 mS/cm. In each case, the starch and the fibrous material were cooked together and then mixed with the pulp. Paper was then made with a Kemira pilot paper machine (type: fourdrinier paper machine; production width: 30 cm; headbox concentration: 0.25-0.5%, circular distributor; wire section with vacuum foils; double press, double felted press section; oil heated drying cylinders; machine speed: 2.5 m/min) and tested.

CMT Burst Resulting Index SCT Strength Breaking Blend Total starch (in Index (in Index (in Length (in Trial type ratio addition amount Nm²/g) Nm/g) kPam²/g) m) Blank — — — 0.81 13.7 1.310 2952 Starch (ref) — 1% 1% 0.84 14.5 1.400 3171 Wheat fiber/Starch 1:4 1.25%   1% 0.94 15.8 1.520 3293 4:1 5% 1% 0.95 15.7 1.620 3631 Soy fiber/Starch 1:4 1.25%   1% 0.87 13.8 1.370 3350 3:2 2.5%   1% 0.91 13.6 1.430 3395 4:1 5% 1% 0.96 14.5 1.490 3619

Further tests were performed, this time with 100% recycled OCC paper with a conductivity value of 11 mS/cm (i.e. in a polluted system). In each case, the starch and the fibrous material were cooked together and then mixed with the pulp.

CMT Burst Resulting Index SCT Strength Breaking Blend Total starch (in Index (in Index (in Length (in Trial type ratio addition amount Nm²/g) Nm/g) kPam²/g) m) Blank — — — 0.82 12.8 1.317 3002 Starch (ref) — 1% 1% 0.89 12.9 1.400 3074 Wheat fiber/Starch 1:4 1.25%   1% 0.92 13.6 1.340 3299 4:1 5% 1% 0.95 13.9 1.570 3568

In all cases, the starch used was of the type C*Bond 05946 available from Cargill Incorporated, with a degree of substitution of 0.042, the wheat fiber was milled wheat bran with an average particle size of 43 μm and the soy fiber was milled soy hulls with an average particle size of 75 μm.

Methods

CMT is measured according to the following standard method: DIN EN ISO7263.

SCT is measured according to the following standard method: DIN 54518

Burst Strength is measured according to the following standard method: Mullen DIN 53141 Part 1 (Tappi 403-OM-85)

Tear Strength (Breaking Load and Breaking Length) is measured according to the following standard method: DIN EN ISO 1924-2 

1. A composition for use as a strengthening agent in the wet-end of papermaking, comprising a surface modified, non-wood plant fiber and a starch component.
 2. The composition of claim 1, wherein at least part of the starch component is linked to the plant fiber.
 3. The composition of claim 1, wherein the plant fiber is anionic.
 4. The composition according to claim 1, wherein the plant fiber is selected from the group consisting of: cereal fibers, seed fibers, legume fibers, fruit fibers, algae fibers and mixtures of two or more thereof.
 5. The composition of claim 1, wherein the plant fiber is selected from the group consisting of: a milled plant fiber, a heat treated plant fiber, a chemically modified plant fiber, a coated plant fiber and any combination thereof.
 6. The composition of claim 1, wherein the plant fiber has an average particle size of 30-500 μm.
 7. The composition of claim 1, wherein the starch component is selected from the group consisting of a cationic and an anionic starch component.
 8. The composition of claim 1, wherein the starch component is cold water soluble.
 9. (canceled)
 10. A method of producing a composition for use as a strengthening agent in the wet-end of papermaking, comprising: providing a composition comprising a plant fiber and a starch component; and surface modifying at least the plant fiber.
 11. A method of producing a composition for use as a strengthening agent in the wet-end of papermaking, comprising: providing a composition comprising a plant fiber; surface modifying the plant fiber; and mixing the modified plant fiber composition with a starch component.
 12. A process for producing paper products, comprising: bringing a composition comprising a surface modified, non-wood plant fiber and a starch component into contact with a cellulosic pulp; and producing a paper product from the pulp obtained in step (a).
 13. A paper product produced by the process of claim
 12. 14. The paper product according to claim 13, comprising surface modified, non-wood plant fibers and cellulose fibers in a weight ratio of 1:500 to 1:4.
 15. The process of claim 12, further comprising increasing the dry strength of the paper product.
 16. The composition of claim 1, wherein the plant fiber has an anionicity of 10 to 2000 μeq/G.
 17. The process of claim 12, wherein the ceLlulosic pulp comprises recycled cellulose fibers.
 18. The process of claim 12, wherein bringing the composition into contact with the cellulosic pulp increases the dry strength of the paper product.
 19. The process of claim 12, wherein the cellulosic pulp has a conductivity level above 4 mS/cm. 